From dd214733544427587a95f66dbf3adff072568990 Mon Sep 17 00:00:00 2001 From: Michael Bommarito Date: Thu, 21 May 2026 10:45:17 -0400 Subject: [PATCH] Bluetooth: L2CAP: reject BR/EDR signaling packets over MTUsig net/bluetooth/l2cap_core.c:l2cap_sig_channel() accepts BR/EDR signaling packets up to the channel MTU and dispatches each command without enforcing the signaling MTU (MTUsig). A Bluetooth BR/EDR peer within radio range can send a fixed-channel CID 0x0001 packet that is larger than MTUsig and contains many L2CAP_ECHO_REQ commands before pairing. In a real-radio stock-kernel run, one 681-byte signaling packet containing 168 zero-length ECHO_REQ commands made the target transmit 168 ECHO_RSP frames over about 220 ms. Impact: a Bluetooth BR/EDR peer within radio range, before pairing, can force 168 ECHO_RSP frames from one 681-byte fixed-channel signaling packet containing packed ECHO_REQ commands. Define Linux's BR/EDR signaling MTU as the spec minimum of 48 bytes and reject any larger signaling packet with one L2CAP_COMMAND_REJECT_RSP carrying L2CAP_REJ_MTU_EXCEEDED before any command is dispatched. The Bluetooth Core spec wording for MTUExceeded says the reject identifier shall match the first request command in the packet, and that packets containing only responses shall be silently discarded. Linux intentionally deviates from that prescription: silently discarding desynchronizes the peer because the remote stack never learns its responses were dropped, and locating the first request command requires walking command headers past MTUsig, i.e. processing bytes from a packet we have already decided is too large to process. We therefore always emit one reject and use the identifier from the first command header, a single fixed-offset byte read. The unrestricted BR/EDR signaling parser and ECHO_REQ response path both trace to the initial git import; no later introducing commit is available for a Fixes tag. Cc: stable@vger.kernel.org Suggested-by: Luiz Augusto von Dentz Link: https://lore.kernel.org/r/20260518002800.1361430-1-michael.bommarito@gmail.com Link: https://lore.kernel.org/r/20260520135034.1060859-1-michael.bommarito@gmail.com Link: https://lore.kernel.org/r/20260521000555.3712030-1-michael.bommarito@gmail.com Assisted-by: Claude:claude-opus-4-7 Assisted-by: Codex:gpt-5-5-xhigh Signed-off-by: Michael Bommarito Signed-off-by: Luiz Augusto von Dentz --- include/net/bluetooth/l2cap.h | 1 + net/bluetooth/l2cap_core.c | 46 +++++++++++++++++++++++++++++++++++ 2 files changed, 47 insertions(+) diff --git a/include/net/bluetooth/l2cap.h b/include/net/bluetooth/l2cap.h index 5172afee5494..e0a1f2293679 100644 --- a/include/net/bluetooth/l2cap.h +++ b/include/net/bluetooth/l2cap.h @@ -33,6 +33,7 @@ /* L2CAP defaults */ #define L2CAP_DEFAULT_MTU 672 #define L2CAP_DEFAULT_MIN_MTU 48 +#define L2CAP_SIG_MTU 48 /* BR/EDR signaling MTU */ #define L2CAP_DEFAULT_FLUSH_TO 0xFFFF #define L2CAP_EFS_DEFAULT_FLUSH_TO 0xFFFFFFFF #define L2CAP_DEFAULT_TX_WINDOW 63 diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c index 45b175399e8d..c4ccfbda9d78 100644 --- a/net/bluetooth/l2cap_core.c +++ b/net/bluetooth/l2cap_core.c @@ -5643,6 +5643,15 @@ static inline void l2cap_sig_send_rej(struct l2cap_conn *conn, u16 ident) l2cap_send_cmd(conn, ident, L2CAP_COMMAND_REJ, sizeof(rej), &rej); } +static inline void l2cap_sig_send_mtu_rej(struct l2cap_conn *conn, u8 ident) +{ + struct l2cap_cmd_rej_mtu rej; + + rej.reason = cpu_to_le16(L2CAP_REJ_MTU_EXCEEDED); + rej.max_mtu = cpu_to_le16(L2CAP_SIG_MTU); + l2cap_send_cmd(conn, ident, L2CAP_COMMAND_REJ, sizeof(rej), &rej); +} + static inline void l2cap_sig_channel(struct l2cap_conn *conn, struct sk_buff *skb) { @@ -5655,6 +5664,43 @@ static inline void l2cap_sig_channel(struct l2cap_conn *conn, if (hcon->type != ACL_LINK) goto drop; + /* + * Bluetooth Core v5.4, Vol 3, Part A, Section 4: the BR/EDR + * signaling channel has a fixed signaling MTU (MTUsig) whose + * minimum and default is 48 octets. Section 4.1 says that on + * an MTUExceeded command reject the identifier "shall match + * the first request command in the L2CAP packet" and that + * packets containing only response commands "shall be + * silently discarded". + * + * Linux intentionally deviates from that prescription: + * + * 1. Silently discarding desynchronizes the peer. The + * remote stack never learns its responses were dropped, + * so any state machine waiting on a paired response + * stalls until its own timer fires. + * + * 2. Locating "the first request command" requires walking + * command headers past MTUsig, i.e. processing bytes + * from a packet we have already decided is too large to + * process. + * + * Reject every over-MTUsig signaling packet with one + * L2CAP_REJ_MTU_EXCEEDED command reject. The reject's + * reason field is what tells the peer that the whole packet + * was discarded; the identifier value is informational, so + * we use the identifier from the first command header, a + * single fixed-offset byte read. + */ + if (skb->len > L2CAP_SIG_MTU) { + u8 ident = skb->data[1]; + + BT_DBG("signaling packet exceeds MTU: %u > %u", + skb->len, L2CAP_SIG_MTU); + l2cap_sig_send_mtu_rej(conn, ident); + goto drop; + } + while (skb->len >= L2CAP_CMD_HDR_SIZE) { u16 len; -- 2.47.3